This proposal describes a three-year training program to develop an independent research career in endocrine physiology. The candidate is an instructor and attending in pediatric endocrinology at the Children's Hospital of Philadelphia (CHOP), with an M.D.-Ph.D. in molecular and cell biology. He extended his training engaged in intensive translational research supported by the Department of Pediatrics Ruth L. Kirschstein National Research Service Award (T32), the Pediatric Endocrine Society (PES) Research Fellowship and the Endocrine Society Clinical Scholars Award and is currently supported by the Division of Endocrinology and Diabetes Career Development Award in Pediatric Diabetes Research (K12), a career development award from the Clinical Practice Enhancement and Anesthesia Research Foundation and the PES Clinical Scholar Award. The proposed research will enhance our understanding of vitamin D metabolism and physiology. Vitamin D insufficiency is common and is thought to reflect insufficient sunlight-dependent synthesis or intake of vitamin D. Low 25-hydroxyvitamin D (calcidiol - 25(OH)D) reduces bone mineralization and impairs calcium and phosphorus homeostasis and is associated with obesity, type 2 diabetes, type 1 diabetes and early mortality. 25-hydroxylation of vitamin D is believed to occur only in the liver through a constitutive enzymatic process. Recent studies, however, suggest the existence of extrahepatic CYP2R1 expression, and I have recently described regulation of liver CYP2R1 expression and activity in aged and obese mice. These results raise three intriguing possibilities with important implications: 1) 25-hydroxylation is regulated, which would provide a mechanism whereby some conditions alter serum 25(OH)D independent of supply; 2) common variation in the CYP2R1 gene gives rise to heterogeneity in 25-hydroxylase activity, which would explain variation in individual supplementation response; and 3) there is physiologically relevant CYP2R1 outside the liver, thus, non-hepatic diseases may affect 25(OH)D in unpredicted, tissue-specific ways. This work will examine these possibilities. This proposal will address critical gaps in our understanding of vitamin D homeostasis, specifically the heretofore unrecognized dynamic regulation of 25-hydroxylation of vitamin D by CYP2R1, and the potential role of the testis as an auxiliary organ for production of 25(OH)D. Additionally, these studies address the unmet need to identify patients at greatest risk of developing disorders related to low 25(OH)D at an early stage using genetic profiling rather than biochemical screening. In sum, the proposed studies and training plan build on the applicants previous work, leverage an optimal infrastructure for training in vitamin D physiology and pathophysiology and bioinformatics, and offer a clear mentored path for continued transition to independent research. The proposed research will be carried out under the mentorship of Michael A. Levine, M.D. and Hakon Hakonarson M.D., Ph.D. Dr. Levine is a leader in the field of vitamin D physiology and is a professor of Pediatrics and the director of The CHOP Bone Health Center (BHC), which includes a substantial research arm. Dr. Hakonarson is a world leader in dissecting the genetic contributors to disease and is an Associate Professor of Pediatrics and the head of the Center for Applied Genomics (CAG). They have each mentored numerous successful Postdoctoral fellows. An advisory committee of talented and invested clinician-scientists has been assembled to guide career development and science. The environment of CHOP, Penn, the BHC and the CAG provides extensive resources and intellectual expertise. This is an ideal training setting to develop the skill set to continue the transition to an independent career as a physician-scientist.